An official in Hungary says a toxic sludge spill, which has killed four people, has reached the edge of Europe's second longest river, the Danube.

Officials say the sludge has reached the Danube, one of Europe's longest rivers. An official also told reporters the toxic spill has wiped out the entire ecosystem of the small River Marcul, which was first contaminated.

Hungarian Prime Minister Victor Orban visited one of the villages hit by the toxic sludge. He described the spill as a "serious ecological catastrophe".

"What I have seen is terrible, simply terrible," Orban said. "This is the first, and probably the most ever (that is) happened, this kind of ecological tragedy that has ever happened here in Hungary."

Reservoir of toxic mud bursts

A dam that was holding back a reservoir of toxic mud from an aluminum plant burst Monday. At least four people were killed and more than 100 injured as the sludge swept into nearby villages.

Corrosive elements in the mud can burn the skin and irritate eyes. The red sludge is a byproduct of bauxite, a basic material for manufacturing aluminum.

State of emergency

The government declared a state of emergency in three counties on Tuesday and has opened a criminal inquiry into the spill.

Joe Lowry from the International Federation of the Red Cross has been to the disaster site. Villagers described to him what the mud looked like as it poured into their village.

"Some people described it like a mini tsunami," said Lowry. "It covered two kilometers from the plant to the village in about 30 minutes, a wall of mud two meters high."

Ecological catastrophe

He says this is not only an ecological catastrophe, but also a human tragedy for the tight-knit Hungarian communities. He says the mud wiped out entire homes and livelihoods.

"There was just scarlet red mud as far as the eye could see all around the village of Colontar," Lowry added. "People were standing around still bewildered and shocked, looking at their homes. Many, many houses were completely, I would say, written off, some were almost completely demolished. Agricultural land is just covered with this thick toxic mud. The main bridge in the village was pretty much snapped in two. The river was turned blood red. Really it was a scene of bewilderment and shock and heavy damage."

The environmental group Greenpeace has described the spill as one of the worst three environmental disasters in Europe during the past few decades. The Danube, after leaving Hungary, runs through several European countries. The toxic mud could affect Croatia, Serbia, Romania, Bulgaria, Ukraine and Moldova before flowing into the Black Sea.